Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. An ion implanter which utilizes a combination of beam scanning and target movement is disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
In the beam scanning approach, an ion beam is deflected by a scanning system to produce ion trajectories which diverge from a point, referred to as the scan origin. The scanned beam then is passed through an ion optical element which performs focusing. The ion optical element converts the diverging ion trajectories to parallel ion trajectories for delivery to the semiconductor wafer. Focusing can be performed with an angle corrector magnet or with an electrostatic lens.
The scanning system typically comprises scan plates to deflect the ion beam, and a scan generator for applying scan voltages to the scan plates. The voltages on the scan plates produce an electric field in the region between the scan plates that deflects ions in the ion beam. A scan voltage waveform is typically a sawtooth, or triangular, waveform, which, in combination with wafer movement, produces scanning of the ion beam over the wafer surface.
Uniform implantation of ions across the surface of the semiconductor wafer is an important requirement in many applications. In theory, a scan waveform with a constant ramp rate of voltage or beam position should produce a beam current that is uniform, i.e., constant at all positions. In practice, this never happens because of aberrations in the beam optics, slight changes in the beam shape as the beam is deflected, nonlinearity in the relationship between voltage and beam position, etc. To achieve a desired uniformity over the wafer surface, a uniformity optimization process has been employed in prior art ion implantation systems. A linear scan waveform is initially applied to the scan plates, so that the scan plates sweep the ion beam in one dimension at a constant rate. The uniformity of the scanned ion beam is measured, and the scan waveform is adjusted to cause a change in the ion beam distribution across the semiconductor wafer. The rate at which the scan plates sweep the ion beam across the wafer surface determines a dosage of the ion implantation. The scan waveform is typically piecewise linear. Initially, all segments of the piecewise linear waveform have slopes of equal magnitude. Adjustment of the scan waveform involves adjusting values which define the slopes of each of the piecewise linear segments of the scan waveform. In general, the initial linear scan waveform may not produce a desired uniformity across the semiconductor wafer, and a nonlinear scan waveform may be required. The measurement of beam uniformity and adjustment of the scan waveform are repeated until the desired uniformity is achieved.
A typical user of the ion implantation system may need to set up multiple implants of different ion species at different energies and doses. The setup process is typically repeated for each set of implant parameters. The setup process is typically time consuming and reduces the throughput of the ion implanters.
In some cases, the setup process for ion implanters is automated. The automated process may permit a predetermined number of iterations of the uniformity optimization process wherein the beam uniformity is measured, the scan waveform is adjusted and the beam uniformity is again measured. If the desired uniformity is not achieved in the predetermined number of iterations, the optimization process is terminated. Accordingly, a parameter known as success rate is associated with the automated uniformity optimization process. The process is considered a success if the desired uniformity is achieved within the predetermined number of iterations. In practice, even the automated optimization process can be time consuming and detract from ion implanter throughput.
Accordingly, there is a need for improved methods and apparatus for optimizing the uniformity of a scanned ion beam.